This invention relates to high voltage transformers and especially to those used in ignition systems for internal combustion engines. More particularly, the invention relates to a tubular winding form or bobbin for the secondary windings of an ignition transformer wherein the primary windings and ferromagnetic core are located within the winding form.
High voltage transformers for ignition systems in modern internal combustion engines generally include a tubular winding form that receives a ferromagnetic core (generally of laminated construction), primary windings surrounding the core and secondary windings wrapped around the winding form. The transformer is generally capable of producing a secondary voltage of around 30 Kv or more.
The form usually has a plurality of axially spaced annular partitions that define annular chambers therebetween. The turns of the secondary windings are wound in the first chamber at one end until the chamber is filled to a desired level. Then the windings proceed to the next chamber such as by passing the wire through a helical transition slot formed in the respective partition and then filling the next adjacent chamber to the same level. This process is continued until all the chambers are filled progressively from one end to the other. The actual winding of the secondary coil is usually accomplished with automatic coil winding equipment.
In modern ignition systems, wider spark gaps are being used (e.g. such as in the range of 0.05 inches and higher) in order to achieve better fuel economy. As a result, higher sparking voltages are necessary such as voltages in excess of 30 Kv. The ignition coils are thus subject to much greater voltage stress than in the past.
In order to accommodate this, several coils are often used in the system such as one coil for every two spark plugs. In this configuration, one end of the secondary coil is connected to one plug and the opposite end is connected to the other plug which is set to fire at an opposite portion of the engine cycle.
One problem that can occur during operation of modern automotive ignition systems of this type is arcing across adjacent coil turns during collapse of the transformer field at the firing point. The firing or arcing across the spark gap of the plug generates an RF voltage that may be reflected back through the ignition cable to the secondary coil. This high voltage transient or spike may have a frequency of around 10 MHz. The resulting RF energy is quickly dissipated in the first three or four turns of the secondary coil, however, the high RF voltage does present a danger of arcing in there first few turns. In fact, arcing from one end turn to the next frequently does occur, thus resulting in deteriation of the insulation on the conductor and of the dielectric material in which the conductor is embedded.
Testing has been accomplished on these coil ignition systems in nitrogen atmosphere pressure vessels under conditions that simulate actual engine operation and with the voltage level adjusted to provide optimum sparking. The tests verify that the RF voltage spikes generated causes deteriation of the insulation of the first few turns of the coil and thus premature coil failure.
The frequency and magnitude of the reflected RF signal is a function of the sparking voltage and the size of the spark gap.
It has been suggested that a solution to the problem is to enlarge the secondary coil form or bobbin to provide greater spacing between the end turns. The spacing would be sufficient to eliminate arcing. While this may be an effective solution, the enlargement of the coil form is often not possible because of the criticality of space for the various components in the engine compartment of the vehicle and in particular, the ignition system components.
The coil form or bobbin of the present invention reduces the difficulties indicated above and affords other features and advantages heretofore not obtainable.